const easeInOutQuad = function (t: number, b: number, c: number, d: number) {
  t /= d / 2
  if (t < 1) {
    return c / 2 * t * t + b
  }
  t--
  return -c / 2 * (t * (t - 2) - 1) + b
}

// requestAnimationFrame for Smart Animating http://goo.gl/sx5sts
const requestAnimFrame = (function () {
  return window.requestAnimationFrame
    || (window as any).webkitRequestAnimationFrame
    || (window as any).mozRequestAnimationFrame
    || function (callback) { window.setTimeout(callback, 1000 / 60) }
})()

/**
 * Because it's so fucking difficult to detect the scrolling element, just move them all
 * @param {number} amount
 */
function move(amount: number) {
  document.documentElement.scrollTop = amount;
  (document.body.parentNode as HTMLElement).scrollTop = amount;
  document.body.scrollTop = amount;
}

function position() {
  return document.documentElement.scrollTop
    || (document.body.parentNode as HTMLElement).scrollTop
    || document.body.scrollTop
}

/**
 * @param {number} to
 * @param {number} duration
 */
export function scrollTo(to: number, duration: number) {

  return new Promise<void>((resolve) => {
    const start = position()
    const change = to - start
    const increment = 20
    let currentTime = 0
    duration = (typeof (duration) === 'undefined') ? 500 : duration
    const animateScroll = function () {
      // increment the time
      currentTime += increment
      // find the value with the quadratic in-out easing function
      const val = easeInOutQuad(currentTime, start, change, duration)
      // move the document.body
      move(val)
      // do the animation unless its over
      if (currentTime < duration) {
        requestAnimFrame(animateScroll)
      } else {
        resolve()
      }
    }
    animateScroll()
  })
}
